Items shipped by air typically are loaded first onto specially configured pallets or into specially configured containers. In the airfreight industry, such pallets and containers are commonly referred to as Unit Load Devices (“ULDs”). ULDs are available in various sizes, shapes and capacities and are typically loaded with cargo at locations other than the immediate vicinity of an aircraft. Once a ULD is loaded with cargo, the ULD is weighed, transferred to the aircraft and loaded onto the aircraft through a doorway or hatch using a conveyor ramp, scissor lift or the like. Once inside the aircraft, a ULD is moved within the cargo compartment to its final stowage position. Multiple ULDs are brought onboard the aircraft and each is placed in its respective stowed position. Once the aircraft reaches its destination, the ULDs are unloaded from the aircraft in a manner that is the reverse of the loading procedure.
To facilitate movement of a ULD within an aircraft cargo compartment, the deck of an aircraft cargo compartment typically includes a number of raised roller elements. These roller elements often include elongated roller trays that extend longitudinally along the length of the cargo deck. The ULDs sit atop these roller elements, and the roller elements facilitate rolling movement of the ULDs within the cargo compartment. Cargo decks are also commonly equipped with a plurality of power drive units (“PDUs”). PDUs are electrically powered rollers that can be selectively energized to propel or drive a ULD in a desired direction over the roller elements of a cargo deck.
Generally, PDUs can be one of two basic types, fixed or steerable. A fixed PDU is secured to a cargo deck structure such that the powered drive roller can only rotate in forward and aft directions within a cargo hold. Such fixed PDUs are typically installed at cargo deck locations remote from an aircraft cargo door, where movement of a ULD can be substantially limited to the forward and aft directions. A steerable PDU (sometimes referred to as a freighter common turntable or FCT) provides for additional directions of rotation of the drive roller. For example, the drive roller may be mounted to a rotatable frame or turntable that can be selectively oriented to align the drive roller in a desired direction within a cargo hold. Similar to a fixed PDU, a steerable PDU can be configured to lift and retract the drive roller between an active raised position and an inactive retracted position. Steerable PDUs are usually installed at cargo deck locations that are proximate to an aircraft cargo door, where a ULD may require movement in a direction other than the forward and aft directions as the ULD is being loaded or unloaded. Because the steerable PDU is configured to rotate within a confined, vertical space, consideration may be given to routing the electrical cabling within the PDU to save space and reduce the potential for inadvertent entanglement of the cabling or grounding of wires due to inadvertent wear.